1. Field of the Invention
The present invention relates to an automatic gain control method and an automatic gain control device adapted for maintaining the level of an amplified analog signal within a proper range when processing the input analog signal from a microphone, line or the like through conversion of the the same into a digital signal.
2. Description of the Related Art
In processing an analog signal inputted from a microphone or the like, audio distortion and so forth are generated in the sound if the level of the analog signal fails to be within a predetermined range. In order to eliminate such a disadvantage, it is generally customary to execute automatic gain control for adjusting any excessive level of the analog signal automatically to a an adequate level.
An exemplary automatic gain control device known heretofore is formed of a bipolar integrated circuit as a whole, and it comprises a detector for detecting the level of an input signal by first rectifying the input signal via a diode and then smoothing the same through a combination of a capacitor and a discharge resistor, and a variable gain amplifier (VGA) for varying the gain in accordance with the signal level detected by such a detector.
In an exemplary known system which converts an input analog signal into a digital signal and outputs the same, there is employed an automatic gain control device for first converting the analog signal into a digital signal by an A-D converter and then varying the gain digitally by a digital amplifier.
In the recent digital technology, it is strongly demanded that an automatic gain control device be constituted of a MOS integrated circuit having various merits in comparison with a bipolar integrated circuit, inclusive of a smaller number of required component parts, a reduced number of production processes, a higher integration density and a lower power consumption.
However, in the automatic gain control system which executes analog-to-digital conversion of an input analog signal and then varies the gain digitally, the analog signal input circuit cannot be placed under control, so that it becomes necessary to employ an amplifier having a sufficiently wide frequency band and an A-D converter. Consequently, some difficulties are unavoidable in manufacturing an automatic gain control device in the form of a MOS integrated circuit.